Implement background-origin for @page backgrounds.
Since the @page background is painted at the page container fragment,
whereas borders and padding are part of the page border box child
fragment, some extra work is required.
Paginated fragment structure documented here:
https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/layout/paginated_root_layout_algorithm.h;l=24-61;drc=01ab59ae08a38a361da7dac41e36f387f6600ed5
Bug: 40286153
Change-Id: I778c40a0c6d3cecd48c7f5d32a4f9071a5e618bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5828871
Commit-Queue: Morten Stenshorne mstensho@chromium.org
Reviewed-by: Fredrik Söderquist fs@opera.com
Cr-Commit-Position: refs/heads/main@{#1350103}